Cholesteric flakes

ABSTRACT

The invention relates to cholesteric polymer flakes obtainable from a chiral polymerizable mesogenic material, to methods of manufacturing such cholesteric flakes, to the use of certain chiral and achiral polymerizable compounds with one or more terminal polymerizable groups for the manufacturing of such flakes and to the use of such cholesteric flakes as effect pigments in spraying or printing inks or paints or colored plastics for different applications, especially for automotive use, cosmetic products and security applications.

FIELD OF THE INVENTION

The invention relates to cholesteric polymer flakes obtainable from achiral polymerizable mesogenic material. The invention also relates tomethods of manufacturing such cholesteric flakes.

The invention further relates to the use of certain chiral and achiralpolymerizable compounds with one or more terminal polymerizable groupsfor the manufacturing of such flakes.

The invention also relates to the use of such cholesteric flakes aseffect pigments in spraying or printing inks or paints or coloredplastics for different applications, especially for automotive use,cosmetic products and security applications.

BACKGROUND OF THE INVENTION

Cholesteric liquid crystals exhibit a helically twisted molecularorientation resulting in special optical properties. When a cholestericLC is irradiated with unpolarized light, interaction of the helixstructure with incident light of a selected wavelength will result inreflection of 50% of its intensity as circularly polarized light of agiven handedness (left-handed or right-handed according to thehandedness of the helix) while the other 50% are transmitted ascircularly polarized light of the opposite handedness. The wavelength λof the reflection maximum depends on the pitch p of the helix and theaverage refractive index n of the cholesteric LC material according tothe following formula:

λ=n.p

Since the color effect of cholesteric optical materials is based onselective light reflection and not on absorption like in conventionaldyes or pigments, extraordinary color-properties can be obtained likefor example higher color saturation, wider color range and iridescentappearance. These materials exhibit a unique reflection pattern, becausethe reflected wavelength will change if the incident light propagatesthrough the cholesteric LC at an angle to the direction of the helixaxis.

However, to achieve good color properties when applied in inks orpaints, a uniform alignment of the cholesteric LC with the orientationof the helix axis parallel to the viewing direction is required.Furthermore, low molar mass cholesteric LC's are best used in mostapplications in the liquid state if they are confined to small capsulesor droplets. In this case the temperature dependence of the reflectedwavelength is another problem. On the other hand polymeric cholestericLC's which are used in the solid state have to be aligned above theirglass transition or melting temperature respectively which requires hightemperatures. Both embodiments are therefore inconvenient for productionand limited in their applications.

By making flakes or platelets of prealigned cholesteric polymer materialseveral problems of the prior art can be circumvented. To produce suchflakes a cholesteric polymer material is coated onto a substrate andaligned to achieve uniform orientation of the helical axis normal to thesurface. The film is then cured and ground to yield small flat flakeswhich can be dispersed e.g. in a transparent binder for the use as inksor paints. These inks can be used at room temperature without the needof further alignment.

PRIOR ART

Such polymer flakes have been described earlier. In U.S. Pat. No.5,364,557 flakes based on cholesteric LC polysiloxanes are disclosed.However, the alignment of an LC polymer as described there is difficultto achieve and has to be carried out above the glass transition, whichrequires high temperatures (120 to 150°C.) and optionally auxiliaryalignment means such as electric or magnetic fields.

Patent application WO 94/22976 describes flakes made by coating twoseparate films of cholesteric LC polysiloxanes which are aligned at hightemperatures, optionally crosslinked and subsequently laminated togetheror are coated on different sides of a base polymer plate. As analternative low molar mass cholesteric LC's with high melting points aredescribed which have to be cooled down rapidly after alignment to obtainan oriented glass. However, these methods require high temperaturechanges and, as the application is confined to laminae of cholestericmaterials, imply a complicated production process with many subsequentsteps.

Furthermore, both documents describe the preferred use of prefabricatedcholesteric LC side chain polysiloxanes. As it is known to the skilledin the art such polymers are usually synthesized by attaching mesogenicside chains in a polymer analogous addition reaction to a polysiloxanebackbone which has been polymerized in advance. Since the opticalproperties and thus the color appearance of the polymer flakes aredepending mainly on the ratio and the chemical structure of themesogenic side chains, they are already determined in the polymer priorto the flake preparation. On the other hand the mechanical properties ofthe flakes are heavily influenced by the chain length and the crosslinkdensity of the polymer, which are fixed during synthesis of thepolysiloxane backbone and/or during flake production. It is thereforedifficult to control all physical and material properties of the pigmentflakes obtained in this way.

The German Application DE 4,419,239 describes cholesteric pigment flakesmade of three dimensional chiral polymer networks and containingpolysiloxanes with cholesterol side chains and methacrylate groups ascrosslinking agent However, besides cholesterol no other chiral groupsare disclosed. Furthermore, the polymer material also has to be preparedin two subsequent polymerization steps as described above.

SUMMARY OF THE INVENTION

Thus the aim of this invention is to provide cholesteric flakes for useas pigments that can be made in a very simple manner which also enableseasy and direct control of the optical and mechanical properties of theproduct.

It was now found that this can be achieved by using certainpolymerizable mesogenic materials and a process according to the presentinvention.

The term “flakes” as it is used throughout the claims and thedescription of this invention comprises small size particles withdimensions of 1 μm to 2 mm. For example, these particles can be granulesof a symmetric or unsymmetric shape, or platelets having average lateraldimensions several times larger than the thickness, or mixtures of bothplatelets and granules.

One of the objects of the invention are cholesteric flakes obtainablefrom a chiral polymerizable mesogenic material by a process includingthe following steps:

(a) coating said material onto a substrate which is then optionallycovered by a second substrate,

(b) aligning the coated material into a planar orientation,

(c) curing the aligned material into a polymer film,

(d) removing the polymer film from the substrate, and

(e) grinding it, optionally under cooling.

In a preferred embodiment of the present invention the chiralpolymerizable mesogenic material comprises at least two polymerizablemesogenic compounds exhibiting at least one terminal poymerizable groupthat is linked, optionally via a spacer group, to a mesogenic core andis selected from the following formulae:

CH₂═CW—COO—  I1

WCH═CH—O—  I2

CH₂═CH—Ph—(O)_(n)—I3

in which W denotes H, CH₃ or Cl and n is 0 or 1.

In another preferred embodiment of the invention the chiralpolymerizable mesogenic material comprises at least two polymerizablemesogenic compounds, wherein each of said compounds exhibits apolymerizable group of the formulae I1 to I4 that is different from atleast one other compound.

In another preferred embodiment of the present invention the chiralpolymerizable mesogenic material is comprising at least one achiralpolymerizable mesogenic compound and at least one chiral polymerizablemesogenic compound, wherein at least one of these compounds exhibits twoor more polymerizable groups.

In another preferred embodiment of the invention at least one of thepolymerizable mesogenic compounds is a fumarate.

In another preferred embodiment the achiral polymerizable mesogeniccompound exhibits two or more polymerizable groups.

In another preferred embodiment the chiral polymerizable mesogeniccompound exhibits two or more polymerizable groups.

In another preferred embodiment the chiral polymerizable mesogenicmaterial comprises at least one photoinitiator.

In another preferred embodiment the chiral polymerizable mesogenicmaterial comprises a non mesogenic compound with one or morepolymerizable groups.

In another preferred embodiment the substrate in step (a) is a polyesterfilm.

Yet in another preferred embodiment the film obtained in step (c) has athickness of 4-10 μm.

Another object of the invention is the use of cholesteric flakes asdescribed above as effect pigments for printing inks, spray paints,automotive use, cosmetic products or-colored plastics.

Yet another object of the invention is the use of cholesteric flakes asdescribed above for active and passive optical elements or as pigmentsin inks and paints for security applications.

Other aims of the present invention are immediately evident to theperson skilled in the art from the following detailed description.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of pigment flakes according to example 1 of theinvention taken by means of a Jeol 6300F scanning electron microscopewith a magnification of 1000.

FIG. 2 is a photograph of pigment flakes according to example 4 of theinvention taken by means of a Jeol 6300F scanning electron microscopewith a magnification of 100.

DETAILED DESCRIPTION OF THE INVENTION

The achiral and chiral polymerizable mesogenic compounds are preferablyselected according to the following formula II:

P—(Sp)_(n)—MG—R  II

wherein

P is a polymerizable group selected of formulae I1 to I4,

Sp is a spacer group having 1 to 20 C atoms,

R is H, halogen or cyano or a chiral or achiral organic group that maybe linear or branched or, in compounds exhibiting more than onepolymerizable group, has the meaning given for P—(Sp)_(n)—,

n is 0 or 1, and

MG is a mesogenic or mesogenity supporting group, preferably linked tothe spacer group Sp and the organic group R by an ester or ether groupor a single bond.

In the compounds of formula II P is preferably a vinyl group, anacrylate or methacrylate group, a styrene group or an epoxy group.Especially preferably P is an acrylate or methacrylate group.

Particularly preferred compounds of formula II are those wherein

MG is a mesogenic or mesogenity supporting group, preferably selectedaccording to formula III

 —(A¹—Z¹)_(m)—A²—Z²—A³—  III

wherein

A¹, A² and A³ are independently from each other 1,4-phenylene in which,in addition, one or more CH groups may be replaced by N,1,4-cyclohexylene in which, in addition, one or two non-adjacent CH₂groups may be replaced by O and/or S, 1,4-cyclohexenylene ornaphthalene-2,6-diyl, it being possible for all these groups to beunsubstituted, mono- or polysubstitted with halogen, cyano or nitrogroups or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms whereinone or more H atoms may be substituted by F or Cl,

Z¹ and Z² are each independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂O—,—CH═C—, —CH≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single bond and

m is 0, 1 or 2,

and

R is an alkyl radical with up to 25 C atoms which may be unsubstituted,mono- or polysubstituted by halogen or CN, it being also possible forone or more non-adjacent CH₂ groups to be replaced, in each caseindependently from one another, by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO——OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— in such a manner that oxygenatoms are not linked directly to one another, or alternatively R ishalogen, cyano or has independently one of the meanings given forP—(Sp)_(n)—.

Particularly preferred is a chiral polymerizable mesogenic materialcomprising at least two polymerizable mesogenic compounds at least oneof which is a compound of formula II.

In another preferred embodiment of the invention the polymerizablemesogenic compounds are selected according to formula II, wherein R hasone of the meanings of P—(Sp)_(n)— as given above.

Bicyclic and tricyclic mesogenic compounds are preferred.

Of the compounds of formula I especially preferred are those in which Ris F, Cl, cyano, or optionally halogenated alkyl or alkoxy, or has themeaning given for P—(Sp)_(n)—, and MG is of formula III wherein Z¹ andZ² are —COO—, —OCO—, —CH₂—CH₂—, —CH═CH—COO—, —OCO— CH═CH— or a singlebond.

A smaller group of preferred mesogenic groups of formula III is listedbelow. For reasons of simplicity, Phe in these groups is 1,4-phenylene,Phe L is a 1,4-phenylene group which is substituted by at least onegroup L, with L being F, Cl, CN or an optionally fluorinated alkyl,alkoxy or alkanoyl group with 1 to 4 C atoms, and Cyc is1,4-cyclohexylene.

—Phe—Z²—Phe—  III-1

—Phe—Z²—Cyc—  III-2

—PheL—Z²—Phe—  III-3

—PheL—Z²—Cyc—  III-4

—Phe—Z²—PheL—  III-5

—Phe—Z¹—Phe—Z²—Phe—  III-6

—Phe—Z¹—Phe—Z²—Cyc—  III-7

—Phe—Z¹—Cyc—Z²—Phe—  III-8

—Phe—Z¹—Cyc—Z²—Cyc—  III-9

—Phe—Z¹—PheL—Z²—Phe—  III-10

—Phe—Z¹—Phe—Z²—PheL—  III-11

—PheL—Z¹—Phe—Z²—PheL—  III-12

—PheL—Z¹—PheL—Z²—Phe—  III-13

—PheL—Z¹—PheL—Z²—PheL—  III-14

In these preferred groups Z¹ and Z² have the meaning given in formula Idescribed above. Preferably Z¹ and Z² are —COO—, —OCO—, —CH₂CH₂—,—CH═CH—COO— or a single bond.

L is preferably F, Cl, CN, NO₂, CH₃, C₂H₅, OCH₃, OC₂H₅, COCH₃, COC₂H₅,CF₃, OCF₃, OCHF₂, OC₂F₅, in particular F, Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃and OCF₃, most preferably F, CH₃, OCH₃ and COCH_(3.)

Particularly preferred are compounds wherein MG is selected from thefollowing formulae

wherein L has the meaning given above and r is 0, 1 or 2.

The group

in this preferred formulae is very preferably denoting

furthermore

with L having each independently one of the meanings given above.

R in these preferred compounds is particularly preferably CN, F, Cl,OCF₃, or an alkyl or alkoxy group with 1 to 12 C atoms or has one of themeanings given for P—(Sp)_(n)—.

If R in formula I is an alkyl or alkoxy radical, i.e. where the terminalCH₂ group is replaced by —O—, this may be straight-chain or branched. Itis preferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms andaccordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy,furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, methoxy, nonoxy, decoxy, undecoxy, dodecoxy,tridecoxy or tetradecoxy, for example.

Oxaalkyl, i.e. where one CH₂ group is replaced by —O—, is preferablystraight-chain 2-oxapropyl (═methoxymethyl), 2-(═ethoxymethyl) or3-oxabutyl (═2-methoxyethyl), 2-, 3-, or 4-oxapentyl, 2-, 3-, 4-, or5-oxahexyl, 2-, 3-, 4-, 5-, or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-,6-,7-, 8- or 9-oxadecyl, for example.

In the polymerizable mesogenic compounds of formula II R may be anachiral or a chiral group. In case of a chiral group it is preferablyselected according to the following formula IV:

wherein

X¹ is —O—, —S—, —CO—, —COO—, —OCO—, —OCOO—or a single bond,

Q¹ is an alkylene or alkylene-oxy group with 1 to 10 C atoms or a singlebond,

Q² is an alkyl or alkoxy group with 1 to 10 C atoms which may beunsubstituted, mono- or polysubstituted by halogen or CN, it being alsopossible for one or more non-adjacent CH₂ groups to be replaced, in eachcase independently from one another, by —C≡C—, —O—, —S—, —NH—, —N(CH₃)—,—CO—, —COO—, —OCO—, —OCO—, —S—CO— or —CO—S— in such a manner that oxygenatoms are not linked directly to one another, or alternatively has themeaning given for P—Sp—,

Q³ is halogen, a cyano group or an alkyl or alkoxy group with 1 to 4 Catoms different from Q².

Preferred chiral groups R are 2-butyl (=1-methylpropyl),

2-methylbutyl, 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,

2-propylpentyl, 2-octyl, in particular 2-methylbutyl, 2-methylbutoxy,

2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy,

2-octyloxy, 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl,

2-nonyl, 2-decyl, 2-dodecyl, 6-methoxyoctoxy, 6-methyloctoxy,

6-methyloctanoyloxy, 5-methylheptyloxycarbonyl, 2-methylbutyryloxy,

3-methylvaleroyloxy, 4-methylhexanoyloxy, 2-chlorpropionyloxy,

2-chloro-3-methylbutyryloxy, 2-chloro-4-methylvaleryloxy,

2-chloro-3-methylvaleryloxy, 2-methyl-3-oxapentyl, 2-methyl-3-oxahexyl,

1-methoxypropyl-2-oxy, 1-ethoxypropyl-2-oxy, 1-propoxypropyl-2-oxy,

1-butoxypropyl-2-oxy, 2-fluorooctyloxy, 2-fluorodecyloxy, for example.

In addition, mesogenic compounds of the formula II containing an achiralbranched group R may occasionally be of importance as comonomers, forexample, due to a reduction in the tendency towards crystallization.Branched groups of this type generally do not contain more than onechain branch. Preferred achiral branched groups are isopropyl, isobutyl(═methylpropyl), isopentyl (═3-methylbutyl), isopropoxy, 2-methylpropoxyand 3-methylbutoxy.

In another preferred embodiment R in formula II is denoting a chiralgroup that is selected from the following groups:

an ethylenglycol derivative

wherein R^(c) is an alkyl radical with 1 to 12 C atoms,

or a group based on citronellol.

In another preferred embodiment of the present invention the compoundsof formula II comprise a mesogenic or mesogenity supporting group MGhaving at least one center of chirality. In these compounds MG ispreferably selected according to formula IIIa:

—(A¹—Z¹)_(i)—G  IIIa

wherein

A¹ and Z¹ have the meaning given in formula III

i is 0, 1 or 2,

G is a terminal chiral group, such as for example a cholesteryl group,

a 2,3-dihydrobenzopyran group

wherein Rd is C₁-C₁₂ alkyl or alkoxy and Z is —COO— or —O—CO—, or aterpenoid radical like, for example, menthol,

As for the spacer group Sp in formula II all groups can be used that areknown for this purpose to the skilled in the art.

Preferably Sp in formula II is a group of the formula S—X, wherein X isthe linkage group to the mesogenic group MG and is denoting —O—, —S—,—CO—, —COO—, —OCO—, —OCOO— or a single bond, and S is a linear orbranched alkylene group having 1 to 20 C atoms, in particular 1 to 12 Catoms, in which, in addition, one or more non-adjacent CH₂ groups may bereplaced by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—, —S—CO—, —O—COO—,—CO—S—, —CO—O—, —CH(halogen)—, —CH(CN)—, —CH═CH— or —C≡C—.

Typical groups S are for example —(CH₂)_(o)—, —(CH₂CH₂O)_(r)—CH₂CH₂—,—CH₂CH₂—S—CH₂CH2 or —CH₂CH₂—NH—CH₂CH₂—, with o being an integer from 2to 12 and r being an integer from 1 to 3.

Preferred groups S are ethylene, propylene, butylene, pentylene,hexylene, heptylene, octylene, nonylene, decylene, undecylene,dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene,ethylene-thioethylene, ethylene-N-methyliminoethylene and1-methylalkylene, for example.

In a preferred embodiment of the invention the polymerizable mesogeniccompounds of formula II comprise a spacer group of the formula S—Xwherein S is a chiral group of the formula V:

wherein

Q¹ and Q³ have the meanings given in formula IV, and

Q⁴ is an alkylene or alkylene-oxy group with 1 to 10 C atoms or a singlebond, being different from Q¹.

In particular preferred are compounds of formula II wherein n is 1.

In the event that R is a group of formula P—Sp—, the spacer groups oneach side of the mesogenic core may be identical or different.

In particular preferred are compounds of formula II wherein n is 1.

In another preferred embodiment, the inventive compensator is obtainedby copolymerizing mixtures comprising compounds of formula II wherein nis 0 and compounds of formula II wherein n is 1.

In case of chiral compounds the groups Sp and/or MG and/or R areselected such that they contain a chiral C atom, or alternativelychirality is arising from a group inducing molecular asymmetry, such ase.g. a binaphthalene group with restricted rotation.

Typical examples representing chiral and achiral polymerizable mesogeniccompounds of the formula I can be found in WO 93/22397; EP 0,261,712; DE195,04,224; DE 4,408,171 or DE 4,405,316. The compounds disclosed inthese documents, however are to be regarded merely as examples thatshould not limit the scope of this invention.

Furthermore, typical examples representing achiral and chiralpolymerizable mesogenic compounds are shown in the following list ofcompounds, which is, however, to be understood only as illustrativewithout limiting the scope of the present invention:

In these compounds x and y are each independently 1 to 12, A is a1,4-phenylene or 1,4-cyclohexylene group, R¹ is halogen, cyano or anoptionally halogenated alkyl or alkoxy group with 1 to 12 C atoms and L¹and L² are each independently H, F, Cl, CN, or an optionally halogenatedalkyl, alkoxy or alkanoyl group with 1 to 7 C atoms.

The reactive mesogenic compounds disclosed in the foregoing and thefollowing can be prepared by methods which are known per se and whichare described in the documents cited above and, for example, in standardworks of organic chemistry such as, for example, Houben-Weyl, Methodender organischen Chemie, Thieme-Verlag, Stuttgart. Furthermore, detailedmethods of preparation can be found for example in D. J. Broer et al.,Makromol. Chem. 190, 2255 (1989) or in the patent applications WO22/3397 or DE 195,04,224.

To induce the cholesteric phase behavior in the chiral polymerizablemesogenic material, for example a mixture comprising an achiral nematicand a chiral nematic polymerizable compound can be used. The chiralnematic compound brings about the helically twisted cholesteric phasestructure. Since the pitch of the cholesteric helix is depending on thechemical constitution and the concentration of the chiral compound, thewavelength of the reflection maximum and therewith the color propertiesof the flakes can be controlled directly in the production process justby varying the type and the ratio of the chiral mesogenic compound. Thustailor-made pigment flakes with the desired colors can be prepared.

Besides the above mentioned components, the mixture may comprise one ormore other suitable components such as, for example, catalysts, light-or temperature-sensitive initiators, stabilizers, co-reacting monomersor surface-active compounds. It is alternatively possible to add, forexample, a quantity of up to 20% by weight of a nonpolymerizableliquid-crystalline material to adapt the optical properties of theproduct. It is also possible to add up to 20% of a non mesogeniccompound with one or more polymerizable functional groups to increasecrosslinking.

In a preferred embodiment of the present invention, the chiralpolymerizable mesogenic material comprises the following components

A2) an achiral polymerizable mesogenic compound having two polymerizablefunctional groups,

B) a chiral polymerizable mesogenic compound having one polymerizablefunctional group,

C) a photoinitiator.

D) optionally a non-mesogenic polymerizable compound having two or morepolymerizable functional groups,

Particularly preferred is a chiral polymerizable mesogenic materialaccording to this preferred embodiment, comprising

a) 10-85%, preferably 20-75% by weight of component A2,

b) 10-90%, preferably 15-85% by weight of component B,

C) 0.01-5%, preferably 0.02-3% by weight of component C,

d) 0-20%, preferably 1-15% by weight of component D.

In another preferred embodiment, the chiral polymerizable mesogenicmaterial comprises the following components

A1) at least one achiral polymerizable mesogenic compound having onepolymerizable functional group,

A2) an achiral polymerizable mesogenic compound having two polymerizablefunctional groups,

B) a chiral polymerizable mesogenic compound having one polymerizablefunctional group,

C) a photoinitiator.

Particularly preferred is a chiral polymerizable mesogenic materialaccording to this preferred embodiment, wherein component A1 comprisesone to six, preferably one to three achiral polymerizable mesogeniccompounds having one polymerizable functional group.

Further preferred is a chiral polymerizable mesogenic material accordingto this preferred embodiment, comprising

a1) 15-85%, preferably 20-75% by weight of component A1,

a2) 5-80%, preferably 10-65% by weight of component A2,

b) 5-80%, preferably 15-70% by weight of component B,

c) 0.01-5%, preferably 0.02-3% by weight of component C.

Further preferred are chiral polymerizable mesogenic materialscomprising the components A1 and C, and optionally the components A2, Band D as described above, together with at least one chiralpolymerizable mesogenic compound having two polymerizable functionalgroups,

The ability of a chiral compound to induce a cholesteric structure witha helical twist of a certain pitch in a nematic host is called itshelical twisting power (HTP). If a material with a high HTP is used,only a small amount is sufficient to achieve reflection of visiblelight. In this case it is not necessary that the pure chiral compoundexhibits a liquid crystal phase. Only in the mixture with the achiralmesogenic compound a liquid crystal phase should be achieved.

The mixture of the achiral and chiral polymerizable mesogenic compoundsis coated onto a substrate, aligned and cured into a polymer film. As asubstrate for example a polyester (PET) film can be used. To achieveuniform alignment with planar orientation, i.e. orientation of the helixaxes normal to the surface of the coated mixture, the film can besheared for example by means of a doctor's blade. In another preferredembodiment, a second PET layer is laminated on top of the coatedmaterial. In this case, the shearing caused by putting the twosubstrates together is sufficient to give good alignment.

The alignment is carried out in the cholesteric phase of the mixture ofthe mesogenic compounds prior to polymerization. Therefore alignment ofa high quality can be achieved considerably easier than for a coatedpolymer film as described in prior art due to the lower viscosity of theunpolymerized material. The application of electric or magnetic fieldsis not necessary.

Furthermore, since mixtures of polymerizable mesogenic monomers normallyexhibit broad nematic or cholesteric mesophase ranges with relativelylow melting temperatures, the film can be aligned and cured attemperatures below 100° C., preferably between 30 and 80° C.

Due to the temperature dependency of the cholesteric pitch the variationof the curing temperature leads to flakes with different reflectionmaxima and is therefore another way to control the color properties ofthe flakes, in addition to variation of the ratio of the chiral andachiral polymerizable mesogenic compounds.

In the curing process the polymerizable groups of the aligned materialreact to form a crosslinked polymer film. With propagatingpolymerization the material becomes glassy and the helical orientationis frozen in. The polymerization can be carried out for example byexposure to UV light with the help of a photoinitiator that decomposesunder irradiation to produce free radicals that start the polymerizationreaction. In another preferred embodiment a cationic photoinitiator isused that photocures with cations instead of free radicals. Thepolymerization may also be started by an initiator that decomposes whenheated above a certain temperature.

To exclude oxygen that may inhibit the free radical polymerization, asecond PET layer may be laminated on top of the coated material, oralternatively the curing can be carried out under a nitrogen atmosphere.In the latter case shearing of the mesogenic material prior topolymerization is necessary to cause sufficient alignment of thecholesteric phase. When using a cationic photoinitiator oxygen exclusionis not needed, but water should be excluded.

These methods, however are only to be understood as examples that shouldnot limit the scope of the invention. The person skilled in the art caneasily find other suitable ways to carry out the polymerization.

Since the mixture may contain both polymerizable components with one(monofunctional) and with two or more polymerizable groups(multifunctional), polymerization and crosslinking are carried out inthe same process. This is in contrast to prior art that describes theuse of cholesteric LC polymers that may optionally be crosslinked in aseparate step or of non-polymerizable low molar mass cholesteric LC's,but gives no hint to the use of multifunctional polymerizable compounds.

By varying the concentration of the multifunctional mesogenic or nonmesogenic components the crosslink density and thereby the productproperties such as glass transition temperature, which is also importantfor the temperature dependence of the optical properties, thermal andmechanical stability or solvent resistance can be tuned easily.According to the desired application flexible or brittle films can bemade. A higher brittleness is desirable in particular when the polymerfilm is subsequently ground to small flakes.

A high brittleness can also be achieved by using compounds with morethan two polymerisable groups which may be mesogenic or non mesogenic.Typical examples for non mesogenic monomers with more thantwo-polymerizable groups are trimethylpropanetrimethacrylate orpentaerythritoltetraacrylate.

Flakes can be formed by grinding the cured polymer film, for example bymeans of a pestle and mortar or by using a mechanized grinder or mill.By additional cooling to temperatures below 0° C. the polymerbrittleness is increased and grinding is made easier. The resultingpowder is then sieved to give pigment flakes of the desired size.

A preferred method to produce flakes of spherical shape with dimensionssmaller than 100 μm is grinding with a pestle and mortar by hand or in amechanized mortar mill.

Another method to produce more or less spherical flakes is by millingthe polymer film in a ball mill. Depending on the size and the weight ofthe balls, particles with average dimensions of less than 100 μm, inparticular of 5 to 10 μm can be obtained.

Another preferred method is milling the polymer film under cooling in ablade mill. This produces a powder of platelet shaped flakes withlateral dimensions from several hundreds of microns to 1 to 2 mm. Theseflakes can subsequently be ground further in a mortar to give plateletswith lateral dimensions smaller than 100 μm.

Cooling of the sample during grinding or milling can be achieved forexample by using a carbon dioxide/acetone bath. Another preferred methodof cooling is the addition of dry ice powder or liquid nitrogen to thesample.

In some embodiments it is preferable to add an antistatic agent whenmilling the polymer material to avoid agglomeration of the particles.

Apart from the method described above, there are further preferredmethods to produce chiral polymer flakes according to the invention:

In another preferred embodiment the flakes are made by coating thechiral polymerizable mesogenic material onto a substrate which containsshallow indentations with a diameter of 10 to 100 μm, preferably 20 to50 μm and a depth of 3 to 20 μm, preferably 4 to 10 μm. In this case theact of coating causes sufficient shear to give uniform alignment. Inorder to increase the quality of the alignment the material mayadditionally be sheared for example by means of a doctor's blade or byapplying a second substrate on top of the coated material as describedabove.

In yet another preferred embodiment the chiral polymerizable mesogenicmaterial is gravure printed in the shape of small droplets onto asubstrate, for example a polyester web, using a gravure printing plateto leave droplets with a thickness of 3 to 20 μm, preferably 4 to 10 μmand a diameter of 10 to 100 μm, preferably 20 to 50 μm. The act ofprinting causes sufficient shear to give uniform alignment, however,here also the material may be additionally aligned by shearing with forexample a doctor 's blade or by applying a second substrate on top ofthe droplets.

Another preferred method to produce cholesteric flakes comprisesspraying of the chiral polymerizable mesogenic material into an N₂atmosphere to give small droplets with a diameter of 10 to 100 μm, whichare cured by irradiation with strong UV light. The cured droplets maysubsequently be ground to make smaller flakes.

Another preferred method is to coat the chiral polymerizable mesogenicmaterial onto a rotating drum, align by a knife edge, cure byirradiation with UV light and scrape off the cured polymer to yieldsmall flakes.

In another preferred method the chiral polymerizable mesogenic materialis coated onto a rotating drum containing dimples with a depth of 2 to20 μm, preferably 3 to 10 μm and a diameter of 10 to 100 μm, preferably20 to 50 μm, cured by UV irradiation and peeled off the drum.

In another preferred method the chiral polymerizable mesogenic materialis coated onto a rotating drum containing stripes that are 2 to 20 μm,preferably 3 to 10 μm deep and 10 to 100 μm, preferably 20 to 50 μmacross, aligned and cured as described above. After this the stripes areground into fragments of the desired size.

In another preferred method an emulsion of the chiral polymerizablemesogenic material in an immiscible liquid is made and the droplets arepolymerized by heating or UV irradiation.

In another preferred method a surfactant is added to the chiralpolymerizable mesogenic material and N₂ gas blown in to make a foamwhich is polymerized, scraped off and ground.

Another preferred method uses a solid particle, preferably carbon blackor graphite dispersed in a solution of the chiral polymerizablemesogenic material and two solvents. Solvent 1 does not dissolve thechiral polymerizable mesogenic material but solvent 2 does. Solvent 2 isboiled off and the precipitating chiral polymerizable mesogenic materialforms a coating over the carbon particle which is then polymerized. Thismethod produces particularly bright flakes.

In yet another preferred method the chiral polymerizable mesogenicmaterial is extruded under pressure through one or more slots with awidth of 2 to 20 μm, preferably 3 to 10 ,μm, whereby the shearingproduces good uniform alignment. The film is cured in an N₂ atmosphere.

The flakes obtained by the above mentioned methods have dimensions ofseveral microns. It is also possible, however, to chose the processparameters so that flakes with lateral dimensions of 500 μm to 1.5 mmare obtained. These flakes show particularly striking color effects andare preferred in certain applications.

For the use in inks and paints, the cholesteric pigment flakes can bedispersed in a transparent binder or fluid, or incorporated intoplastics, depending on the application.

For some applications, it is preferable to use mixtures of flakes withdifferent reflection maxima.

Applications

The cholesteric polymer flakes can be used as effect pigments inspraying or printing inks or paints or colored plastics for decorativeapplications like for example cosmetic products. Other important fieldsof application are automotive use, active or passive optical elements,like e.g. optical films such as polarizers or compensators, and thesecurity sector, for example in false-proof security labels such as IDcards, credit cards or tickets.

As explained in detail above, a considerable advantage of the inventionlies in the fact that the optical and the mechanical properties of thepigment flakes can all be controlled in one and the same process simplyby changing the type and the concentration of the chiral and achiral,mono- and multifunctional mesogenic polymerizable compounds. Thus thepigment flakes can be tailored appropriately for the desiredapplication.

The complete disclosure of all applications, patents and publicationsmentioned hereinbefore and hereinafter is introduced into thisapplication by way of reference.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following examples are, therefore, to beconstrued as merely illustrative and not limitative of the remainder ofthe disclosure in any way whatsoever.

In the foregoing and following examples, all temperatures are set forthuncorrected in degrees Celsius and unless otherwise indicated, all partsand percentages are by weight The following abbreviations are used toillustrate the liquid crystalline phase behavior of the compounds:

C=crystalline; N=nematic; S=smectic; Ch=cholesteric; I=isotropic. Thenumbers between these symbols indicate the phase transition temperaturesin degree Celsius.

EXAMPLES Example 1

A mixture is formulated consisting of

50% of compound A

49% of compound B and

1% of the commercially available photoinitiator Irgacure 651 (by CibaGeigy AG, Basel, Switzerland).

The mixture exhibits the mesophase behavior C 44-47 Ch 53 I and areflection wavelength of 542 nm at 40° C.

The mixture is coated at 40° C. onto a web of polyester. A secondpolyester web is laminated over the top to cause alignment of thecholesteric phase and exclude oxygen. The aligned mixture between thetwo polyester webs is then cured with UV light of a power of 5 mW/cm² togive a green polymer film of 6-8 μm thickness. The polymer film ispeeled off the substrates for further processing.

5 g of the polymer film are mixed with 500 g dry ice powder and groundto small flakes using a pestle and mortar. After sieving with a 100 μmsieve, the flakes which passed through the sieve are collected anddispersed into a nitrocellulose binder. The dispersion is sprayed onto ablack metal substrate to give a coating that shows a bright iridescentgreen color at normal incidence and reflects blue light at glancingangles of incidence.

FIG. 1 shows an SEM photograph of pigment flakes of example 1 obtainedfrom a Jeol 6300F electron microscope at an acceleration voltage of 20kV with a magnification of 1000. The flakes have a globular shape with adiameter ranging from about 3 to about 20 μm.

Example 2

A mixture is formulated consisting of

54% of compound A

45% of compound B and

15 1% of Irgacure 651

The mixture reflects red light and has the mesophase behavior C 51Ch51i.

The mixture is coated, cured and ground using a pestle and mortar asdescribed in example 1 to give red polymer flakes with a reflectionwavelength of λ=606 nm.

Example 3

A mixture formulated of

47% of compound A

48% of compound B

30 4% of trimethylpropanetrimethacrylate (as non-mesogenic crosslinkingagent) and

1% of Irgacure 651

reflects green light and has a monotropic cholesteric phase when cooleddown from the isotropic phase with the mesophase behavior C 41-56 (Ch 36i).

The mixture is coated, cured and ground using a pestle and mortar asdescribed in example 1 to give green flakes with a reflection wavelengthof λ=522 nm.

Example 4

A polymer film produced as described in example 1 is milled in a blademill from Ika. The resulting powder is mixed with dry ice, ground in amortar and sieved through a 100 μm sieve. The resulting flakes aredispersed and sprayed on a black substrate as described above to give acoating which shows bright iridescent green color at normal incidenceand reflects blue light at glancing angles of incidence.

FIG. 2 shows an SEM photograph of the flakes of example 4 obtained froma Jeol 6300F electron microscope at an acceleration voltage of 5 kV anda distance of 25 mm with a magnification of 100. The flakes have aplatelet shape with an average thickness of about 10 μm and lateraldimensions of about 100 to 200 μm.

What is claimed is:
 1. A flake product of a cholesteric crosslinkedpolymer, prepared by a process comprising: (a) coating a chiralpolymerizable mesogenic material onto a substrate which is thenoptionally covered by a second substrate, (b) aligning the coatingmaterial into a planar orientation, (c) curing the aligned coatingmaterial into a crosslinked polymer film, (d) removing the crosslinkedpolymer film from the substrate, and (e) grinding it, optionally whilecooling, wherein the chiral polymerizable mesogenic material comprisesat least two different polymerizable mesogenic compounds each having atleast one terminal polymerizable group that is linked, optionally via aspacer group, to a mesogenic core wherein the polymerizable group isselected from the following formulae: CH₂═CW—COO—  I1 WCH═CH—O—  I2CH₂═CH—Ph—(O)_(n)—I3

in which W denotes H, CH₃ or Cl and n is 0 or
 1. 2. The product of claim1, wherein the chiral polymerizable mesogenic material comprises atleast two different polymerizable mesogenic compounds each having atleast one terminal polymerizable group that is linked, optionally via aspacer group, to a mesogenic core wherein the polymerizable group isselected from the following formulae: CH₂═CW—COO—  I1 WCH═CH—O—  I2CH₂═CH—Ph—(O)_(n)—I3

in which W denotes H, CH₃ or Cl and n is 0 or
 1. 3. The product of claim1, wherein the at least two polymerizable mesogenic compounds havepolymerizable group of the formulae I1 to I4 that is different from eachother.
 4. A flake product of a cholesteric crosslinked polymer, preparedby a process comprising: (a) coating a chiral polymerizable mesogenicmaterial onto a substrate which is then optionally covered by a secondsubstrate, (b) aligning the coating material into a planar orientation,(c) curing the aligned coating material into a crosslinked polymer film,(d) removing the crosslinked polymer film from the substrate, and (e)grinding it, optionally while cooling, wherein the chiral polymerizablemesogenic material comprises at least one achiral polymerizablemesogenic compound and at least one chiral polymerizable mesogeniccompound wherein at least one of these compounds exhibits two or morepolymerizable groups.
 5. A flake product of a cholesteric crosslinkedpolymer, prepared by a process comprising: (a) coating a chiralpolymerizable mesogenic material onto a substrate which is thenoptionally covered by a second substrate, (b) aligning the coatingmaterial into a planar orientation, (c) curing the aligned coatingmaterial into a crosslinked polymer film, (d) removing the crosslinkedpolymer film from the substrate, and (e) grinding it, optionally whilecooling, wherein at least one polymerizable mesogenic compound is afumarate.
 6. The product of claim 4, wherein the achiral polymerizablemesogenic compound exhibits two or more polymerizable groups.
 7. Theproduct of claim 4, wherein the chiral polymerizable mesogenic compoundexhibits two or more polymerizable groups.
 8. The product of claim 1,wherein the chiral polymerizable mesogenic material comprises at leastone photoinitiator.
 9. A flake product of a cholesteric crosslinkedpolymer, prepared by a process comprising: (a) coating a chiralpolymerizable mesogenic material onto a substrate which is thenoptionally covered by a second substrate, (b) aligning the coatingmaterial into a planar orientation, (c) curing the aligned coatingmaterial into a crosslinked polymer film, (d) removing the crosslinkedpolymer film from the substrate, and (e) grinding it, optionally whilecooling, wherein the chiral polymerizable mesogenic material comprises anon mesogenic compound with one or more polymerizable groups.
 10. Theproduct of claim 1, wherein in the preparation process the substrate instep (a) is a polyester film.
 11. The product of claim 1, wherein in thepreparation process the film obtained in step (c) has a thickness of4-10 μm.
 12. A printing ink, spray paint, cosmetic product or coloredplastic composition which comprises a flake product of a cholestericcrosslinked polymer according to claim
 1. 13. An active or passiveoptical element which comprises a flake product of a cholestericcrosslinked polymer according to claim
 1. 14. An ink or paint used for asecurity application which comprises a flake product of a cholestericcrosslinked polymer according to claim
 1. 15. A flake product of acholesteric crosslinked polymer containing a polymerized mesogeniccompound of the formula II: P—(Sp)_(n)—MG—R  II wherein P is apolymerizable group selected from formulae I1 to I4, Sp is a spacergroup having I to 20 C atoms, R is H, halogen or cyano or a chiral orachiral organic group that may be linear or branched or, in compoundsexhibiting more that one polymerizable group, has the meaning given forP—(Sp)_(n)—, n is 0 or 1, and MG is a mesogenic or mesogenity supportinggroup of formula III —(A¹—Z¹)_(m)—A²—Z²—A³—  III wherein A¹, A² and A³are independently from each other 1,4-phenylene in which, one or more CHgroups are optionally replaced by N, 1,4-cyclohexylene in which, one ortwo non-adjacent CH₂ groups are optionally replaced by O and/or S,1,4-cyclohexenylene or naphthalene-2,6-diyl, all of which areunsubstituted, mono- or polysubstituted with halogen, cyano or nitrogroups or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms whereinone or more H atoms are optionally substituted by F or Cl, Z¹ and Z² areeach independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —CH═CH—,—C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single bond and m is 0, 1 or
 2. 16.The product of claim 15, wherein the compound of the formula II is achiral compound.
 17. A method for preparing a flake product of acholesteric crosslinked polymer which comprises: (a) coating a chiralpolymerizable mesogenic material onto a substrate which is thenoptionally covered by a second substrate, (b) aligning the coatingmaterial into a planar orientation, (c) curing the aligned coatingmaterial into a crosslinked polymer film, (d) removing the crosslinkedpolymer film from the substrate, and (e) grinding it, optionally whilecooling, wherein the chiral polymerizable mesogenic material comprisesat least two different polymerizable mesogenic compounds each having atleast one terminal polymerizable group that is linked, optionally via aspacer group, to a mesogenic core wherein the polymerizable group isselected from the following formulae: CH₂═CW—COO—  I1 WCH═CH—O—  I2CH₂═CH—Ph—(O)_(n)—I3

in which W denotes H, CH, or Cl and n is 0 or
 1. 18. The product ofclaim 2, wherein at least one polymerizable mesogenic compound is acompound of the formula II: P—(Sp)_(n)—MG—R  II wherein P is apolymerizable group selected from formulae I1 to I4, Sp is a spacergroup having 1 to 20 C atoms, R is H, halogen or cyano or a chiral orachiral organic group that may be linear or branched or, in compoundsexhibiting more that one polymerizable group, has the meaning given forP—(Sp)_(n)—, n is 0 or 1, and MG is a mesogenic or mesogenity supportinggroup of formula III —(A¹—Z¹)—A²—Z²—A³—  III wherein A¹, A² and A³ areindependently from each other 1,4-phenylene in which, one or more CHgroups are optionally replaced by N, 1,4-cyclohexylene in which, one ortwo non-adjacent CH₂ groups are optionally replaced by O and/or S,1,4-cyclohexenylene or naphthalene-2,6-diyl, all of which areunsubstituted, mono- or polysubstituted with halogen, cyano or nitrogroups or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms whereinone or more H atoms are optionally substituted by F or Cl, Z¹ and Z² areeach independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —CH═CH—,—C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single bond and m is 0, 1 or
 2. 19.The product of claim 18, wherein the compound of the formula II is achiral compound.
 20. The product of claim 19, wherein the R group informula II is a chiral group of the formula IV:

wherein X¹ is —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or a single bond, Q¹is an alkylene or alkylene-oxy group with 1 to 10 C atoms or a singlebond, Q² is an alkyl or alkoxy group with 1 to 10 C atoms which isunsubstituted, mono- or polysubstituted by halogen or CN, optionally oneor more non-adjacent CH₂ groups are replaced, in each case independentlyfrom one another, by —C≡C—, —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—,—OCO—, —OCO—O—, —S—CO— or —CO—S— in such a manner that oxygen atoms arenot linked directly to one another, or alternatively has the meaninggiven for P—(Sp)_(n)—, Q³ is halogen, a cyano group or an alkyl oralkoxy group with 1 to 4 C atoms different from Q².
 21. The product ofclaim 16, wherein the R group in formula II is a chiral group of theformula IV:

wherein X¹ is —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or a single bond, Q¹is an alkylene or alkylene-oxy group with 1 to 10 C atoms or a singlebond, Q² is an alkyl or alkoxy group with 1 to 10 C atoms which isunsubstituted, mono- or polysubstituted by halogen or CN, optionally oneor more non-adjacent CH₂ groups are replaced, in each case independentlyfrom one another, by —C≡C—, —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—,—OCO—, —OCO—O—, —S—CO— or —CO—S— in such a manner that oxygen atoms arenot linked directly to one another, or alternatively has the meaninggiven for P—(Sp)_(n)—, Q³ is halogen, a cyano group or an alkyl oralkoxy group with 1 to 4 C atoms different from Q².
 22. A flake productin platelet-shaped form of claim 1 having lateral dimensions of lessthan 100 μm.
 23. The flake product of claim 4, wherein the chiralpolymerizable mesogenic material comprises the components A2 an achiralpolymerizable mesogenic compound having two polymerizable functionalgroups, B a chiral polymerizable mesogenic compound having onepolymerizable functional group, C a photoinitiator, D optionally anon-mesogenic polymerizable compound having two or more polymerizablefunctional groups.
 24. The flake product of claim 4, wherein the chiralpolymerizable mesogenic material comprises the components A1 at leastone achiral polymerizable mesogenic compound having one polymerizablefunctional groups, A2 an achiral polymerizable mesogenic compound havingtwo polymerizable functional groups, B a chiral polymerizable mesogeniccompound having one polymerizable functional group, C a photoinitiator.25. The flake product of claim 4, wherein the chiral polymerizablemesogenic material comprises the components A1, C, at least one chiralpolymerizable mesogenic compound having two polymerizable functionalgroups, and optionally components A2, B and D A1 at least one achiralpolymerizable mesogenic compound having one polymerizable functionalgroups, A2 an achiral polymerizable mesogenic compound having twopolymerizable functional groups, B a chiral polymerizable mesogeniccompound having one polymerizable functional group, C a photoinitiator,D optionally a non-mesogenic polymerizable compound having two or morepolymerizable functional groups.
 26. The flake product of claim 15,wherein MG is selected from groups of formulae III-1 to III-14—Phe—Z²—Phe— III-1 —Phe—Z²—Cyc— III-2 —PheL—Z²—Phe—  III-3—PheL—Z²—Cyc—  III-4 —Phe—Z²—PheL—  III-5 —Phe—Z¹—Phe—Z²—Phe—  III-6—Phe—Z¹—Phe—Z²—Cyc—  III-7 —Phe—Z¹—Cyc—Z²—Phe—  III-8—Phe—Z¹—Cyc—Z²—Cyc—  III-9 —Phe—Z¹—PheL—Z²—Phe—  III-10 —Phe—Z¹—PheZ2—PheL—  III-11 —PheL—Z¹—Phe—Z²—PheL—  III-12—PheL—Z¹—PheL—Z²—Phe—  III-13 —PheL—Z¹—PheL—Z²—PheL— III-14.
 27. Theflake product of claim 15, wherein MG is selected from the followingstructures:

wherein L has the meaning given above and r is 0, 1 or
 2. 28. The flakeproduct of claim 16, wherein MG is of formula IIIa —(A¹—Z¹)_(i)—G  IIIawherein A¹ is 1,4-phenylene where one or more CH groups are optionallyreplaced by N and, additionally, where one or two non-adjacent CH₂groups are optionally replaced by O and/or S; 1,4-cyclohexenylene; ornaphthalene -2,-6-diyl; each of these being unsubstituted, mono- orpoly-substituted with halogen, cyano or nitro groups or alkyl, alkoxylor alkanoyl groups having I to 7 C atoms, optionally one or more H atomssubstituted by F or Cl; Z¹ is —COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂—O—,—CH═CH—, —C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single bond; i is 0, 1 or2; and G is cholesteryl, 2,3-dihydrobenzopyran or terpenoid group. 29.The flake product of claim 15, wherein Sp is S—X, with S being linear orbranched alkylene with 1-12 C atoms and X being —O—, —S—, —CO—, —COO—,—OCO—, —OCOO—, or a single bond.
 30. The flake product of claim 15,wherein Sp is S—X, with S being a group of formula V

wherein Q¹ is an alkylene or alkylene-oxy group with 1 to 10 C atoms ora single bond, Q³ is a halogen, cyano group or alkyl or alkoxy groupwith 1 to 4 C atoms, and Q⁴ is an alkylene or an alkylene-oxy group with1 to 10 C atoms or a single bond, being different from Q¹.
 31. The flakeproduct of claim 1, wherein the chiral polymerizable mesogenic materialis aligned and cured in the cholesteric mesophase below 100° C.
 32. Theflake product of claim 31, wherein the chiral polymerizable mesogenicmaterial is aligned and cured in the cholesteric mesophase between 30and 80° C.
 33. A flake product of claim 1, having in spherical flakeform average particle dimensions of less than 100 μm.
 34. An ink orpaint comprising a flake product of claim 1 dispersed in a transparentbinder or fluid.
 35. An automobile paint comprising a flake product ofclaim 1 dispersed in a transparent binder or fluid.
 36. A securitylabel, credit card or ticket, comprising a flake product of claim
 1. 37.An optical film as a polarizer or compensator, comprising a flakeproduct of claim
 1. 38. The flake product of claim 1, which is in theform of platelets.
 39. The flake product of claim 1, which is in theform of platelets having lateral dimensions of from several hundreds ofmicrons to 2 mm.